The green fluorescent protein (GFP) from jelly fish of the genus Aequoreus has been isolated and characterized by several workers, and the gene that encodes this protein has now been cloned and expressed in numerous organisms to serve as a marker for aspects of cell physiology. We are interested in converting the fluorescence of GFP to an electron scattering stain so that fine structural studies can be used to complement and extend the work in labs that use GFP as a reporter for protein position. Previous work in our lab has led to only limited success in photoconverting GFP by fixing cells that express it, bleaching their fluorescence with either an argon ion laser or a mercury vapor lamp in the presence of diaminobenzadine (DAB), followed by further fixation with OsO4, then embedding, sectioning and electron microscopy. We are now pursuing two alternative approaches: screening for mutant alleles of the GFP that photoconvert better than the wild type; and finding a small molecule, like eosin, that can serve as an acceptor for fluorescence resonance energy transfer (FRET) of the energy from excited GFP. Acceptors that are in the immediate vicinity of GFP will then be excited and can oxidize DAB. This approach circumvents the difficulty of getting free radicals from GFP photobleaching out of its ~-barrel without their reacting with the protein's many functional groups.